This invention relates to precision indicator gauges utilized in machine shops and the like. So-called dial indicator gauges are well known in the art for measuring displacement, that is, the distance that a workpiece (or the like) moves with respect to a given point. The gauges are delicate and include many intricate parts that are carefully assembled and adjusted. They are also used in conjunction with suitable attachments for enabling machinists to precisely center a drill press or mill with respect to a circular hole in a workpiece. The dial indicator, when used as a test indicator to show relative movement, is coupled by means of a mechanical linkage arrangement to the tool chuck and adjusted such that the sensing foot of the indicator engages the periphery of the hole in the workpiece that is to be centered. The machinist manually rotates the tool spindle a small amount and notes any deflection of the indicator needle. He then determines an approximate correction to be made to the workpiece supporting table to bring the hole closer to the center of the spindle. After the adjustment is made, the spindle is again rotated slightly while the needle of the test indicator gauge is observed. Further adjustments are made to bring the center of the hole in the workpiece on center with the tool spindle. When the test indicator needle shows substantially zero deflection as the spindle is rotated through 360.degree., the machinist is assured that the spindle is centered with respect to the hole in the workpiece. This is a somewhat tedious trial and error method that requires the machinist to observe the indicator needle movement (or lack thereof) in a number of circumferential orientations. Since the gauge rotates with the spindle, the machinist must crane his neck into very awkward positions to observe the needle. Needless to say, the technique is very time consuming. While the test indicator in the above illustration is a precision device, it is used only to indicate movement and not to measure the amount of movement.
As mentioned, dial indicators are precision instruments and are therefore somewhat delicate. During normal machine shop use, the indicators are often subjected to inadvertent rough handling and it is quite common to require periodic recalibration and replacement of broken parts to restore them to proper operation. In addition to the high initial cost of the dial indicator, repair or reconstruction costs are also high. It would be of great benefit to a machinist to have a low cost and an accurate indicator gauge that is ruggedly built to withstand use in a machine shop environment. It would also be a boon to a machinist to have an indicator gauge that could be used to simply and effectively determine when the spindle of a rotary tool is centered with respect to a circular hole in a workpiece.
The indicator gauges of the present invention use an incompressible liquid, such as mercury, to measure movement by displacement of the liquid in a closed tube or channel, preferably formed in a molded gauge body. The position of the liquid in the channel is determined by movement of a piston that in turn is driven by movement of a sensing device that contacts the workpiece. The body of the indicator is preferably fabricated of clear plastic material with the surface in which the channel is formed being of a contrasting color to enhance the visibility of the liquid, which is preferably mercury. A circular version of the liquid indicator of the invention, with a spiral groove in its face, and a cylindrical straight line version are disclosed. The liquid gauges constructed in accordance with the invention are accurate, low cost, simple to manufacture and use, and are extremely rugged.